1. Field of the Invention
The present invention relates to a data signal quality evaluation method using high speed sampling, suitable for use when sampling an optical or electrical data signal with a predetermined bit rate and displaying an eye-diagram and measuring signal quality.
2. Description of the Related Art
A first example of a conventional optical signal quality evaluation apparatus is shown in FIG. 10 (see reference document “Handbook of ELECTRONIC TEST EQUIPMENT (Section 5–8. SAMPLING OSCILLOSCOPE), pp. 184–189, JOHN D. LENK, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1971”, reference document “Modeling of the HP-1430A Feedthrough Wide-Band (28-ps) Sampling Head, SEDKI M. RIAD, IEEE Transactions on Instrumentation and Measurement, Vol. IM-31, No. 2, June 1982, pp. 110–115”, for example). This conventional optical signal quality evaluation apparatus comprises an opto-electric conversion device 101 which converts an optical signal with a bit rate of f0 (bit/s) into an electric intensity modulated signal, a clock extraction device 102 which extracts a clock from the electric intensity modulated signal, a sampling clock generation device which generates a sampling clock with a repetition frequency of f1(Hz)(f1=(n/m)f0+a, where n and m are natural numbers, and a is the offset frequency) synchronized with the clock extracted by the clock extraction device 102, and an electrical signal processing device 104. The electrical signal processing device 104 samples the electric intensity modulated signal input via the clock extraction device 102 in accordance with the sampling clock, and displays a sampled data distribution sequentially based on the obtained sampled electrical signal, thereby obtaining a signal eye-diagram, and evaluates the optical signal quality parameters.
As a second conventional example, resembling the first conventional example described above, there are optical sampling devices using a sampling optical pulse train having a repetition frequency of f1(Hz)(f1(n/m)f0+a, where n and m are natural numbers and a is the offset frequency) and a pulse width sufficiently narrower than a timeslot of an optical signal, and optical sampling devices using a sampling clock (see Japanese Patent No. 2677372, Japanese Patent No. 3239925, reference document “100 Gbit/s optical signal eye-diagram measurement with optical sampling using organic nonlinear optical crystal, H. Takara, S. Kawanishi, A. Yokoo, S. Tomaru, T. Kitoh and M. Saruwatari, Electronics Letters, Vol. 32, No. 24, 21st Nov. 1996, pp. 2256–2258”, for example). These optical sampling devices are provided before the opto-electric conversion device. In these examples, an optical splitter splits the optical signal, and optical sampling is performed using a sampling clock or a sampling optical pulse train synchronized with the clock obtained by performing clock extraction from one of the split optical signals. The sampled optical signal is then converted into a sampled electrical signal by the opto-electric conversion device. The electrical signal processing device then displays sequentially a sampled data distribution based on the obtained sampled electrical signal, thereby obtaining a signal eye-diagram, and evaluates the optical signal quality parameters.
The repetition frequency of the sampling clock in the first conventional example is normally within the range of several dozen to several hundred kHz, and it takes time to obtain a signal eye-diagram which is necessary and sufficient for evaluation, and therefore wander of the optical signal becomes an issue. Consequently, clock extraction was essential. In the second conventional example in which optical sampling is performed by the sampling clock or the sampling optical pulse train, the repetition frequency of the sampling clock or the sampling optical pulse train is approximately 10 MHz, but because it is necessary to perform the electrical signal processing to determine the sampled data distribution sequentially, the effective sampling rate decreases, and it takes time to obtain a necessary and sufficient signal eye-diagram for evaluation, and therefore wander of the optical signal becomes an issue. Consequently, clock extraction was essential.
As described above, because clock extraction is required in both the first and second conventional examples, this presents such problems as increases in the scale of the apparatus, increases in the complexity of the method or apparatus, and increases in the cost of the apparatus. An optical signal monitoring apparatus using asynchronous sampling (see European Patent Application Publication No. EP 0920150 A2, reference document “Optical signal quality monitoring method based on optical sampling, I. Shake, H. Takara, S. Kawanishi and Y. Yamabayashi, Electronics Letters, Vol. 34, No. 22, 29th Oct. 1998, pp. 2152–2154”, for example), is proposed as a third conventional example, as an optical signal monitoring apparatus which does not require clock extraction. However, because this method evaluates an optical signal intensity distribution based on an asynchronous eye-diagram, it still cannot be applied to degradation in the time domain (such as jitter and polarization dispersion).